Scripta Materialia最新文献

Keeping adequate strain hardening to postpone plastic instability to a larger tensile strain is a stiff challenge in alloys with high yield strength. This study showed the feasibility of activating deformation twins (DTs) to ductilize an ultra-strong medium-entropy alloy (MEA) through microstructural design. High content (∼24 %) γ'' precipitates were introduced into a Ni_49.9Fe₃₃Cr₁₀Nb₄Ta₃B_0.1 (at.%) MEA to offer high yield stress. We found that increasing the γ'' precipitate size and spacing successfully activated DTs, contributing to a sustainable strain hardening of the alloy. Accordingly, an ultrahigh tensile yield stress (YS) of 1.55 GPa and ultimate tensile stress (UTS) of 1.7 GPa, along with a fracture elongation of 14.5 % were achieved in the MEA. We further demonstrated that compared to lowering the stacking-fault energy (SFE), increasing γ'' precipitate spacing significantly reduced the critical shear stress for triggering twinning partials in a nanoscale γ/γ'' system.

In this work, the crystallization path and non-isothermal kinetics upon heating of the Zr_59.5Cu_14.4Ni_11.6Al_9.7Nb_4.8 metallic glass were investigated. The devitrification process consists the formation of phases in the sequence of icosahedral quasicrystal (IQ) phase, Ni-containing phases, and Cu-containing phases. Avrami exponents were calculated at various heating rates, providing insights into the non-isothermal crystallization kinetics. The IQ phase and Ni-containing phases are interface-controlled growth, while the Cu-containing phases are diffusion-controlled growth. In addition, a continuous heating transition (CHT) diagram with a heating rate range exceeding six orders of magnitude was constructed, and the crystallization mechanism under different heating rates was revealed. These findings enrich the understanding of crystallization path and kinetics of metallic glass.

There is considerable interest in magnetic materials which also possess good mechanical properties. Hence, the effect of Ti/Al ratio on the microstructure, mechanical and magnetic properties of (FeCoNi)₉₀Ti_10-xAl_x complex concentrated alloys (CCA) was investigated. An increase in the Ti/Al ratio in these CCA enhanced chemical ordering and substantially improved selected mechanical and magnetic properties. As the Ti/Al ratio changed from 10 to 0, the ductility increased from 7.5 to close to 50 %, the saturation magnetization (M_s) increased from 115.2 to 136.7 emu/g, and the coercivity (H_c) decreased from 17.9 to 4.2 Oe. The Fe₃₀Co₃₀Ni₃₀Ti₅Al₅ alloy exhibit higher UTS×EL value than available soft magnetic materials and has relatively higher M_s and lower H_c compared with other CCA. These results provide a methodology to modulate the chemical order in the Fe-Co-Ni system by Al and Ti additions and synergistically tune the mechanical and magnetic properties for high performance rotating electrical machine applications.

Hydrogen-induced variations in mechanical behavior of zirconium alloys impose detrimental influence on nuclear fuel cladding integrity. This work reports a disappearance of intrinsic yield drop in a recrystallized zirconium alloy following hydrogen-charging treatment. Microstructure characterizations reveal that the nano-hydrides precipitation, mediated by second phase particles Zr(Fe,Cr)₂ acting as hydrogen trapping sites, leads to emission of substantial dislocations in α-matrix grains due to strong strain concentrations, as identified by high-angular resolution EBSD. These mobile dislocations preserved at elevated temperatures can maintain the applied plastic strain and impede rapid dislocation multiplication as well, a conclusion validated by comparative analysis of dislocation densities prior to and near yielding stage. These findings are expected to shed light on the underlying mechanisms governing the interaction between hydrogen and microstructural defects in Zr-based nuclear fuel cladding materials.

Oxide Dispersion Strengthened (ODS) steels hold great promise for applications in next generation reactors. Under irradiation, a phase separation α/ α’ can occur within the Fe-Cr matrix of ODS steels that can alter their mechanical properties. This work presents, for the first time, the characteristics of α’ precipitates enhanced by ion irradiation at 400 °C and examines the influence of the implanted ions. Far from the implanted region, α’ is reported in significant density while at the implanted peak, the α’ density is considerably reduced. This suggests that ion implantation either reduces the fraction of α’ phase formed after irradiation or delays considerably its formation. Through atom probe tomography analysis and comparison with existing literature, the low impact of the damage rate and fluence on the α’ formation in ODS steels is highlighted. Interestingly, the efficiency of ballistic mixing of α’ appears to be less pronounced in ODS steels than in Fe-Cr systems.

Achieving damage tolerance in structural materials can be challenging due to the need for both high strength and ductility, which are typically incompatible properties. The common post-processing techniques in thermomechanical machining enable us to fabricate metal materials with distinctive microstructures, thereby enhancing the mechanical properties of the materials. We show that a hierarchical-structured titanium (HST) alloy consisting of belt-like α phase (α_b), submicron-scaled oval α phase (α_o), and nano-scaled secondary α phase (α_s) has been designed by employing precision and user-friendly process routes. The hierarchical microstructure performs high strength while preserving respectable ductility. The ultrahigh strength (σ_YS∼1257 MPa and σ_UTS∼1411 MPa)) can be mainly attributed to the grain boundary strengthening served by hierarchical α phase. Moreover, the unique architecture provides excellent resistance to crack propagation, obtaining a large ductility (20%), making it a highly promising structural material for engineering applications.

Selective leaching accompanied by ferritization is widely observed in the dissolution corrosion of austenitic steels exposed to lead-bismuth eutectic. This study proposes a refined mechanism for selective leaching involving solid-state diffusion followed by dissolution-reprecipitation processes. Pole figure analysis reveals that ferritization follows the Nishiyama-Wassermann orientation relationship, independent of corrosion environment or corrosion behavior. Moreover, austenite reconstruction analysis shows that the growth of ferrite is hindered by the misfits at random high-angle grain boundaries. Hence the above results suggest that ferritization is initially driven by solid-state diffusion. As LBE channels open within the grains, further growth of ferrite is assisted by the dissolution-reprecipitation mechanism. Additionally, the formation of dual-oriented ferrite grains at Σ3 twin boundary is observed, and the selectivity of this formation with respect to grain boundary and ferrite orientation is discussed.

This study presents an innovative deep learning-based surrogate model for the Crystal Plasticity Finite Element (CPFE) method, fundamentally transforming the generation of mechanical properties such as stress-strain curves in the study of crystal plasticity. Stress-strain curves are pivotal in understanding material deformation, elucidating the intricate relationship between a material's structure and its properties. Traditional CPFE methods, though thorough in their analysis, face significant computational challenges, largely due to the complexity of the crystal plasticity framework. The proposed model circumvents this bottleneck by utilizing an autoencoder architecture to learn intermediate data representations, which are then used to predict the plastic component of deformation. This predicted plastic component serves as a foundation for computing stress-strain curves, effectively bypassing the most time-intensive aspect of traditional CPFE methods, the plasticity self-consistency procedure (achieving a 29.3x speed increase without compromising accuracy).

Traditional approaches for improving the mechanical performance of alloys entail modifying interfaces, particularly grain boundaries, with elemental segregation or secondary phases. However, these methods face challenges in concurrently improving the strength, plasticity, and high-temperature softening resistance of alloys. Here, we uncovered that stable isostructural coherent interfaces effectively address these challenges. In the model body-centered cubic (BCC) MoTaVW refractory high-entropy alloy (RHEA) fabricated by mechanical alloying and spark plasma sintering, controlling the sintering temperature enhances the preferential segregation of W at interfaces. This results in a distinct BCC W-enriched nanolayer between micrometer-scale grains. This nanolayer facilitates dislocation slip and prevents grain growth, thereby improving both plasticity and resistance to high-temperature softening. Consequently, the MoTaVW RHEA featuring stable isostructural coherent interfaces achieves an ultrahigh yield strength of 1410 MPa and a plasticity of 22 % at ambient temperature. Even at 1200 °C, it maintains a yield strength of 575 MPa under hot compression.

Nanocrystalline alloys are promising structural materials yet lack thermal stability in many cases. Recent work shows that interface structure has an outsize effect on the thermal behavior of nanostructured alloys. This work focuses on the role of controlled heterophase interface structure in the thermal evolution of model Cu/Nb nanolaminates. We introduce 3D interfaces containing nanoscale heterogeneities in all spatial dimensions between Cu and Nb, forming 3D Cu/Nb. TEM, nanoindentation, and DSC are used in tandem to establish thermal stability and to identify shifts in microstructure as a function of static annealing temperature. 3D interfaces are shown to survive annealing to 300 °C for 1 hr., while 3D Cu/Nb microstructure evolves to form low-density and voided regions correlating to the onset of layer pinch-off between 500 and 600 °C annealing temperatures. A diffusivity- and vacancy energetics-based mechanism is developed to explain void formation driven by 3D interface degradation at elevated temperature.